1. Field of the Invention
The present invention relates to a method for producing L-lysine by fermentation and to an L-lysine producing mutant.
2. Description of the Background
L-lysine, which is important as an additive for livestock feed, and the like has been mainly prepared by fermentation.
In the industrial preparation of L-lysine by fermentation, some technical factors must be considered in order to achieve economic improvement of the process. These factors include, for example, improvements in yield based on sugar, improvements in concentration of L-lysine which accumulates, shortening of culturing time, and the like.
In culturing, it is important to elevate the temperature for culturing. Culturing is generally carried out at an optimum temperature for L-lysine fermentation. Where conventional L-lysine-producing bacteria are used, the temperature is generally between 28.degree. and 35.degree. C. When culturing begins, the heat of fermentation is generated so that when the system is allowed to stand, the temperature of the culture medium increases with the result that the amount of L-lysine which forms seriously decreases. In order to maintain the temperature of the culture medium in the optimum range, it is necessary to mount a heat exchanger in the fermentation tank and cyclize cool water into the tank. In order to obtain cool water, a cooler must be used. However, since the heat of fermentation generated is very large, the amount of electric energy consumed by the cooler is very large. Accordingly, if the culture temperature for fermentation of L-lysine is increased, the cooling load decreases so that economy of L-lysine production in an industrial scale can be improved.
The method described in Korean Patent Publication No. 85-231 (published August 23, 1985) proposes to increase the culture temperature in the fermentation of L-lysine. That is, the method disclosed employs the L-lysine-producing variant TR-3579 (KFCC 10065) which belongs to the genus Corynebacterium, which has resistance to a lysine analogue and resistance to temperature and further requires homoserine, leucine and valine in combination. Culturing of the variant occurs in a medium at a high temperature (37.degree. to 45.degree. C), thereby producing and accumulating L-lysine in the culture medium.
Since the L-lysine-producing variant used in this method is obtained by improving conventional L-lysine-producing bacteria (grown at 30.degree. to 35.degree. C.) by a mutation which results in the production of L-lysine at a high temperature (37.degree. to 45.degree. C.). the variant has complex auxotrophy of homoserine, leucine and valine as described above, because of the several step mutation employed. Therefore, this method is not practical.
Japanese Patent Application Laid-Open No. 58-170487 discloses a method for producing L-lysine by fermentation which comprises culturing a mutant belonging to the genus Corynebacterium, having a reduced pyruvate kinase acivity, which is capable of producing L-lysine and also which has optional resistance to S-(2-aminoethyl)-L-cysteine in a medium thereby producing and accumulating L-lysine in the culture broth and then collecting L-lysine.
The temperature for fermentation in the method is set at 20.degree. to 40.degree. C., but as is assumed from the only working example (Example 1) in the reference where a temperature of 30.degree. C. is adopted, the growth of the bacteria used is not significant at about 40.degree. C. which is the upper limit of the temperature range for fermentation. Marked formation and accumulation of L-lysine are not observed either. This has been confirmed by experimentation. A need therefore continues to exist for an improved method of producing L-lysine by fermentation.